Ventilation important in high tunnels

COLUMBIA, Mo. – University of Missouri Extension regional horticulture specialist James Quinn talked about side ventilation in high tunnels at a recent workshop at MU’s Bradford Research and Extension Center.

Quinn began his work with high tunnels at Mizzou in 2003. High tunnels have become popular in the past decade as a way for produce farmers to extend their growing season and maximize profits.

Quinn said that drop-down sides on high tunnels need more than a single sheet of plastic for ground-to-ground cover and are easier to use. Roll-up sides require only one sheet, but plants near the edge of the high tunnel can be stunted.

“Bigger is better for side ventilation,” Quinn said. High tunnels warm quickly; one vent open downwind is good for less cooling while opening both sides allows for more cooling. He recommended that growers invest in a “max/min” thermometer to aid in keeping the crop within its preferred growing temperature. High tunnels require frequent checking—up to three times per day in variable-weather periods.

Floating row covers offer additional frost protection, but growers should note that temperatures could soar underneath the cover and that higher humidity resulting from the use of row covers could increase the likelihood of disease.

As the growing season progresses, sides may be left open at all times once outside temperatures remain at 60 degrees or higher at night. A shade cloth should be added at the end of May and remain through September, Quinn said. A 50 percent shade cloth improves the quality of tomatoes without reducing yield.

Ventilation in the peak creates a chimney effect that aids cooling in the summer. Full ridge vents are preferable, he said, but add material and labor costs to the high tunnel.

Quinn said frost-damaged tomatoes can recover quickly. Temporary heat can provide relief. Don’t cut lettuce and salad greens immediately after mild frost damage, he said. They will recover on their own.